LATE GLACIAL AND HOLOCENE ALLUVIAL FANS IN BUTTERNUT VALLEY, UPSTATE NEW YORK

A series of alluvial fans rest at the mouth of several tributaries to Butternut Creek, a headwaters drainage to the Susquehanna River in the Appalachian plateau of upstate New York. While the general shape of the fans can be traced from topographic maps, field investigations reveal several geomorphic surfaces at each fan that document changing conditions of deposition, from deposition in direct contact with ice, to modern ice-free conditions. Early Archaic artifacts have informally been reported from these fans. We are beginning a more formal description of the fans, and a detailing of the stratigraphy. We focused our efforts at the mouth of a small second order tributary to Butternut Creek, where we identify an alluvial ice-contact fan forming the highest surface (Fan 3); an alluvial fan deposited on top of stagnant ice at an intermediate level and extending to the center of the valley (Fan 2); and a modern alluvial floodplain and fan (Fan 1) that grade into the active floodplain of Butternut Creek. The highest surface, Fan 3, comprises imbricate cobble-to-boulder size material, viewable in channel cutbanks. Its geomorphic expression is a gentle slightly concave sloping (2-3 degrees) surface that ends abruptly at an angle of repose 15 m scarp at its downstream end. Surface material is silty sand, with frequent cobbles encountered by soil push cores. Fan 2 is the largest surface and extends from the mouth of the tributary to the center of the main valley. It exhibits a somewhat irregular surface but slopes 2-3 degrees, and has the general shape of a fan. Parts of its toe displays angle of repose scarps (up to 7 m high) that border lower-lying wetlands. Other parts of the fan toe are clearly truncated by old meander scars of Butternut Creek. In no location does this surface grade into the modern alluvial plain of Butternut Creek. Fan 1 is the lowest of the fans, and comprises a floodplain and active channel. While this tributary is perennial, it is a disappearing stream at its distal end, where flow spreads out and infiltrates into coarse-grained material. Springs mark the re-emergence of the stream on fine-grained material of the modern alluvial plain. We utilize GPR, soil cores, and outcrop exposures to characterize the fan stratigraphy, and illuminate the nature of contacts between fans.